Printing plate and method of making same



Sept. 27, 1966 STEP (e) B. COHN 3,275,441

PRINTING PLATE AND METHOD OF MAKING SAME Filed March '7, 1963 STEP (0) 1PAL l MOLYBDATE TREATMENT STEP(b) 3 BEFORE REDUCTlON MOLYBDATE TREATMENTAFTER REDUCTION PHOTOSENSITIVE LAYER STEP(C) 5 BERNARD COHN I-NVENTOR.

AGENT T United States Patent 3,275,441 PRINTING PLATE AND METHOD OFMAKING SAME Bernard Cohn, Elmsford, N.Y., assignor to Technilrth Inc.,Mount Vernon, N.Y., a corporation of New York Filed Mar. 7, 1963, Ser.No. 263,479

8.Claims. .(Cl. 96 -75) The present invention relates to planograph'icprinting plates and like lithographic printing surfaces as well as to amethod of producing such plates.

In the lithographic printing art, it is known to provide a metallicsubstrate (e.g. an aluminum plate or aluminumfoil support with ahydrophilic layer) and thereafter to coat this layer with aphotosensitive material. After exposure to light, nonsensitized portionsof this layer may then be rubbed oif or otherwise removed to prepare theplate for use upon a lithographic press. Prior to. this, however, animage developer may be employed to render visible the image formedduring sensitization. I

Many techniques have been proposed heretofore for the preparation ofsuch plates but none have met with much success. is the plate proposedby Jewett et al. (US. Patent No. 2,714,066) which makes use of asilicate layer as the hydrophilic coating. While this plate represents avast improvement over earlier printing layers, it was neverthelesscharacterized by a relatively short press life and a shorter shelf lifethan may be desirable. The problem of shelf life is of particularimportance since modern lithographic processes make use, to a largedegree, of sO-called presensitized plates, one form of which can beproduced in accordance with commonly assigned copending application Ser.No. 183,817, filed March 30, 1962. In such presensitized plates, thehydrophilic surface of the substrate is provided with the photosensitivematerial and stored after drying but prior to exposure. In the latterapplication there is disclosed a system for making use of essentiallynonmetallic substrates and a method of providing such substrates with asuitable hydrophilic layer. As previously mentioned, however, it isdesirable to attain longer shelf life and press life with metal-basedlithographic plates as well.

It is another object of the present invention to provide an improvedlithographic printing plate of this nature.

These objects are attained, in accordance with the invention, by amethod which comprises the steps of treating a metal substrate andpreferably one composed of an amphoteric metal, especially aluminum,with an acidified solution of a molybdate salt to produce a hydrophiliccoating thereof. -It has, however, been discovered that the mere use ofa molybdate treatment does not permit extended shelf life although itdoes provide a suitable hydrophilic printing surface. It has been found,surprisingly, that both the shelf life and the press life of amolybdate-treated aluminum plate can be improved manyfold if, after themolybdate treatment, the plate is exposed to a reducing agent capable ofreducing molybdate ion so that excessive quantities thereof are removedfrom the plate surface. The latter is presumably penetrated by themolybdate solution which forms bonds therewith with the excess beingsubstantially in a non-bonded state and possibly available for affectingthe photosensitive layer deposited upon the hydrophilic surface. Thislayer is, advantageously, formed by polymeric condensation of adiazodiphenylaminosulfate and paraformaldehyde. Other substances of thegeneral type disclosed in US. Patent No. 2,063,631 to Schmidt and Zahn(Example I) are also suitable. It has been found that best results areobtained when the molybdate solution is derived from an alkali metalmolybdate salt and preferably sodium molybdate.

An exception insofar as shelf life is concerned 3,275,441 Patented Sept.27, 1966 The acid employed in conjunction with the sodium molybdate maybe any of the protonic acids although, in some cases, Lewis acids (eg;aluminum chloride) are suitable in addition to carboxylic acids. Theacid, when of the protonic variety, should contain a large anion havinga central atom from Groups 5 or 6 of the Periodic Table; thus arseniousand phosphoric acids are suitable along with sodium biphosphate andsodium bisulfite.

The above and other objects, features and advantages will become morereadily apparent from the following description and examples, referencebeing made to the appended drawing whose sole figure represents a flowdiagram in cross-sectional views illustrating the successive steps a-eof manufacturing a printing plate in accordance with the presentinvention, some of the layers being exaggera-ted in thickness.

In the drawing, there is shown an aluminum substrate 2 whose surface 1is to be rendered hydrophilic. The substrate can be a sheet of aluminumor an aluminum foil having a base of other material although an aluminumcoating on a support will also sufiice for the present purposes. Asindicated at step b the surface 1 is treated with an acidic molybdatesolution to provide a hydrophilic layer 3 which, at the interface 4between this layer and the substrate, apparently bonds to the aluminum.Excess or nonbonded molybdate is removed by treatment with a reducingagent to yield a surface zone 3 whose molybdate content renders thesurface 5 highly hydrophilic without substantial long-term effect upon apresensitizing di-azo layer 6 (step d). This latter photosensitive layercan then be exposed to actinic light through a stencil 'or negativewhereupon sponging removes the noninsolubilized portions of the layer toproduce channels or grooves 7 while the insolubilized portions formlands 8. 'The grooves 7 extend to the hydrophilic surface 5 which isreceptive to water and thus repels the lithographink when moistenedwhile the lands 8' are hydropholic and inkreceptive.

Example I A plate is produced in the aforementioned manner by immersinga sheet of aluminum in a cleaning or pick-ling bath such as hottrisodium phosphate until all surface Oils and grease films have beenremoved. Satisfactory cleaning is attained at a temperature of about C.with a 20% solution of trisodium phosphate after a period of about 5minutes. The sheet is then rinsed in water and immersed in concentratednitric acid to eliminate any scum which may have been formed during thecleaning step. In the absence of such a scum, the nitric-acid scouringcan be eliminated. Y e

The rinsed plate is then treated by immersion with an aqueous solutioncontaining about 0.1 weight percent sodium molybdate and 0.2 volumepercent phosphoric acid at a temperature of about 60 C. fora period of 2minutes. The hydrophilic surface is then rinsed in water and dried andis found to have a coating of the type illustrated in step b with someof the molybdate at the surface beipg nonbonded to the aluminum butadhering to that portion of the coating which is so bonded. Aphotosensitive layer consisting of a condensation polymer ofp-diazodiphenylamine-sulfate and formaldehyde can then be deposited uponthe hydrophilic coating. While the resulting plate is found to have .arelatively long press life, its shelf life is somewhat diminishedapparently by reason of the excess molybdate.

The photosensitive compound can be produced in the manner disclosed inthe Jewett et al. patent, e.g. by mixing 34 parts of the sulfate salt ofp-diazodiphenylamine with 3.25 parts of paraformaldehyde in the presenceof 4.5 parts of anhydrous zinc chloride. The mixture is introduced intosulfuric acid at a temperature below 6 C. with the resulting solutionpoured on ice to, produce a tarry material which is dissolved in water,treated with zinc chloride and precipitated. A water solution of theyellow precipitate forms the light-sensitive coating. The resultingplate has a press life which is greater than that of a silicate-treatedplate.

Example ll The method of Example I is followed with the exception that,after the treatment with molybdate solution and prior to coating withthe light-sensitive compound, the hydrophilic surface is subjected tothe action of a reducing agent. The reducing agent can be an aqueoussolution, containing weight percent thiosulfate, the.

treatment time being about 1 minute at a temperature of 60 C. When theplate is then rinsed, dried and coated 4 t and 5 weight percentsodium:bisulfite, the, latter acting as a protonic acid.

Example XI The method of Example 11 is followed except that themolybdate bath contains 1% by weight sodium molybdate and 3% byvolume'arsenious acid.

In all thesecases long-wearing lithographici plates are produced whoseshelf life in a presensitizedstate far exceeds that possible withearlier treatments.

I claim:

1. A method of making lithographic printing plates,"

comprising the steps of reacting an acidic, molybdate solution with asurface of an aluminum substrate to with the diazo polymer, it has ashelf life much greater than thatof the plate of Example I. Comparativetests indicated that the plate produced according to the instant examplehad a press life almost four times as great as the silicate-treatedplate. The press life of the plates whose preparation is describedhereunder is of a similar order of magnitude.

Example III The procedure of Example II is followed except that themolybdate solution is comprised of 1 weight percent sodium molybdate and10 weight percent sodium bisulfate which acts as a protonic acid.

Example IV The processof Example IIiis followed with the exceptionthatthe molybdate bath thereof is replaced by an aqueous solutioncontaining 0.1% by weight sodium molybdate and 1 'volume percent aceticacid. The molybdate treatment is carried out for 2 minutes at 80 C.

Example. V,

The method of Example H is followed with the exception that themolybdate bath contains 0.5 weight percent sodium molybdate and 5% byweight aluminum chloride, the latter acting as a Lewis acid.

Example VI The steps of Example H are followed withzthe exception thatthe reducing bath is replaced by an aqueous solutioncontaining 1% byweight ferrous sulfate at a temperature of 60 C., the treatmenttimebeing 1 minute.

Example VII Themethod of Example VI is followed with the molybdate bathbeingoomposed of an aqueous solution containing 0.1% by weight sodiummolybdate and 1% by volume nitric acid.

Example VIII The steps of Example II are followed with, however, thesodium molybdate replaced by an equal amount of ammonium molybdate.

Example IX The method of Example II is followed except thatthe molybdatesolution is replaced by an 85% molybdic acid solution.

' Example X The method of Example H is followed with the molyb-' datebath containing 5% by weight sodium molybdate render said surfacehydrophilic; thereafter; treating said surface with an aqueous solutionof a reducing agent selected'from the group consisting of thiosulfateand ferrous salts; and coating :the hydrophilic surface 50 treated witha layer of a photosensitive polymeric diazo substance.

2., A method as defined in claim 1 wherein said molyb-- date solutionincludes a molybdate saltand an acid selected from the group whichconsists of sodium bisulfate,

7 sodium biphosphate, phosphoric acid, nitric acid, ar-- senious acid,acetic acid and aluminum chloride.

3. A method as defined in claim 2 wherein said molybdate salt isselected from the group whichconsists of sodium molybdate and ammoniummolybdate.

4; A method as defined in claim 1 wherein said sol-u-- tion is anaqueous solution of molybdic acid.

5.v A method as defined in claim 1 wherein said reducing agent is anaqueous solution of: a salt" selected from the group which consists offerrous sulfateand 8; A presensitized lithographic printing platecompris-- ing an aluminum substrate havinga surface initially treated.with an acidic molybdate solutionan-d thereafter with an aqueousreducing agentselected from the, group consisting of thiosulfate andferroussalts, and a layer of a photosensitive polymeric diazo compoundoverlying said surface, said compound being a condensation polymer ofdiazodiphenylaminesulfate and paraformaldehyde.-

References Cited by theExaminer UNITED STATES PATENTS Strecker ,1486.15

597,366 1/ 1898 2,816,051 12/1957 Hal-ford 1486.14 2,899,368 8/ 1959Spooner 204-58: 3,030,210 4/1962 Chebiniak 9633 X 3,052,575 9/1962 Crane148-614 3,085,008 4/1963 Case. 96- 33 X 3,201,247 8/1965 Leonard 9633FOREIGN PATENTS 5 815,471 6/1959 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

I. TRAVIS BROWN, Examiner. RQL. STONE, A. D. RICCI, .AssistantExaminers.

8. A PRESENTIZED LITHOGRAPHIC PRINTING PLATE COMPRISING AN ALUMINUMSUBSTRATE HAVING A SURFACE INITIALLY TREATED WITH AN ACIDIC MOLYBDATESOLUTION AND THEREAFTER WITH AN AQUEOUS REDUCING AGENT SELECTED FROM THEGROUP CONSISTING OF THIOSULFATE AND FERROUS SALTS, AND A LAYER OF APHOTOSENSITIVE POLYMERIC DIAZO COMPOUND OVERLYING SAID SURFACE, SAIDCOMPOUND BEING A CONDENSATION POLYMER OF DIAZODIPHENYLAMINESULFATE ANDPARAFORMALDEHYDE.